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In theory, one could probably be made to fit the other. The saucers are of a very similar shape and size. In practice, however, the way that they're joined is very different. The link between the saucer on the Nebula class is almost 50 metres longer.

The Nebula is sometimes thought to be smaller than the Galaxy class.
At least this was the original intention by the designers. Rick
Sternbach writes at TrekBBS: "The Nebula class was supposed to be
smaller than the Galaxy class, IIRC; the saucer shape is similar, but
the bridge module is larger in proportion to the saucer size, as are
the windows. The fact that the Nebula saucer looks like a Galaxy
saucer (shape, colors, various design elements) doesn't exactly help
the situation, but there it is."

Nice answer! Regarding the similarity in appearance as far as saucer, deflector, and nacelles go, my understanding was that they were both made at the same shipyard - Utopia Planitia, Mars. This suggests a lot of common design, possibly by the same engineering team, but not necessarily cross-compatibility on that scale.
– Vanguard3000Apr 1 '18 at 23:00

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@Vanguard3000 - Indeed. And out-of-universe, the Nebula-class was made by kitbashing several Enterprise-D models.
– ValorumApr 1 '18 at 23:07